The present invention relates to a fluxmeter and more particularly to a fluxmeter of the type in which flux is measured by electronically integrating the voltage generated in a test coil over a test period during which the flux linking the coil is changed by an amount to be measured.
Fluxmeters are conventionally constructed employing electronic integrator circuits, usually of the so-called Miller type. In such a circuit, an integrating capacitor is connected from the output of an inverting amplifier to its input so that the output voltage from the amplifier varies as a function of the time integral of any current applied to the input terminal.
In order to make a measurement, the integrating capacitor is momentarily shorted, e.g., by a push button switch, in order to zero the output of the instrument and the flux change to be measured is then imposed on the measuring coil. The value of the integral is then read as a function of the resultant output voltage of the integrator.
Usually the amplifier selected for the integrator is of a type exhibiting a very low level of bias current and a small offset voltage so as to minimize errors in the output signal due to integration of the bias current and offset voltage. In the absence of an input signal, the error signal appears as a residual drift of the output signal. Typically, a compensating current is supplied to the input terminal to at least partially offset any residual drift and this compensating current is manually adjusted to achieve minimum drift in the absence of any input signal. The optimum value of compensating current will, however, typically vary as a function of various influences, e.g., temperature, component aging, and so forth, as is familiar to those skilled in the electronic arts. Thus, in order to achieve an accurate measurement, it is typically necessary to manually readjust the value of compensating current just prior to the taking of each measurement. As will be understood, this adjustment procedure is relatively time consuming since the drift, which is the time integral of only the uncompensated offset current, must reach an observable level before any further adjustment can be made.
Among the several objects of the present invention may be noted the provision of a fluxmeter of improved accuracy and ease of operation; the provision of such a fluxmeter which is of the integrating type and in which a compensating current provided to the main integrating circuit is automatically adjusted to an optimum value; the provision of such an apparatus in which the value of compensating current is automatically adjusted between test periods; and the provision of such apparatus which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.